One shot, One kill

To do the tier 4 missions, do you need to have full tier 4 gear? I have 3 or 4 right now. But when I do "Gates" the first bad guy to hit me either kills me outright or knocks me down so far I can't help the team much. I will admit I'm no expert on this game, so I am guessing I have missed some little nuance type of thing. Or is it just brute force? Do I need more of something?